The invention relates to pressure swing adsorption processes and more particularly to PSA processes for the production of high purity oxygen (e.g. oxygen having a purity of 90-95 vol. % O2). More particularly, the invention is directed towards particular adsorbents for use in PSA processes.
The objective of this invention is to enhance the mass transfer rate of adsorbent materials, particularly those used in PSA. With a fast mass transfer rate, one can have short cycle time and low power consumption and therefore high adsorbent productivity and high process efficiency in PSA systems and processes.
It has been recognized that it is possible to shorten cycle time by reducing particle size of adsorbent aggregates. This recognition has been based upon the assumption that the time needed for adsorbates to travel through the macropores of the adsorbents limits the adsorption/desorption cycle time, i.e. macropore diffusion is the rate limiting step in adsorption processes.
Armond et al. (UK Pat. Appl. GB 2 091 121) disclose a superatmospheric PSA process for air separation in which short cycles ( less than 45 sec) are combined with aggregates of small diameter (0.4 mm to 3.0 mm) to reduce the process power and the size of the adsorption bed. They reported that cycle times of 15 s to 30 s and aggregate diameter of 0.5 mm to 1.2 mm are their preferred choice.
Hirooka et al. (U.S. Pat. No. 5,122,164) also utilized small particles to achieve fast cycles and they devised process cycles with 6, 8 or 10 process steps to improve yield and productivity. They preferred aggregate diameter of 0.8 mm to 1.7 mm and cycle times of 50 s to 60 s.
Very small adsorbent particles (0.1 mm to 0.8 mm) are necessary for the fast cycles and high pressure drop that characterize a special class of processes known as rapid pressure swing adsorption (RPSA). Typical RPSA processes have very short feed steps (often less than 1.0 s) operating at high feed velocities, include a flow suspension step following the feed step and generally have total cycle times less than 20 s (often less than 10 s).
Jones et al teaches that RPSA of single adsorption bed using adsorbent aggregates of 20-120 mesh (0-84mm to 0.125 mm) is able to achieve a cycle time of less than 30 seconds (U.S. Pat. No. 4,194,892). Earls et al teach RPSA air separation using multi-bed cycles using 40 to 120 mesh (0.520 mm to 0.125 mm) aggregates and a cycle time from 0.2 to 18 seconds (U.S. Pat. No. 4,194,891).
Wankat developed a methodology to scale columns according to particle diameter whereby through the use of smaller diameter, one can reduce the volume of adsorbents needed. This is referred to as xe2x80x9cintensificationxe2x80x9d of the sorption process. (P C Wankat, Ind. Eng. Chem. Res. Vol. 26, No. 8, p.1579 1987).
Unfortunately,. however as the diameter of the aggregates decreases, the pressure drop across the bed increases. Further, there is increased potential for fluidization and greater difficulty in particle retention in the bed. The net effect is an undesirable increase in the energy consumption of the process.
Kinetics of sorption in PSA processes has been discussed in texts such as xe2x80x9cPrinciples of Adsorption and Adsorption Processesxe2x80x9d by Ruthven, John Wiley and Son, 1984; and Gas Separation by Adsorption Processes, by Yang, Butterworth, 1987). In these discussions, the kinetic parameter of an adsorbent is defined as a function of the macropore diffusion coefficient, which in turn has been defined as a function of the porosity of the macropore.
Based on these theoretical developments, Moreau et al (U.S. Pat. No. 5,672,195) concluded that an adsorbent should have a kinetic parameter A(k) of at least 0.5 sxe2x88x921 and a porosity of between 0.38 and 0.6. Moreau et al did not address the significant offsetting effects of high porosity, including the fact that increasing the porosity or intraparticle void fraction reduces the overall active adsorbent content of the particle resulting in lower particle density. This in turn increases the volume of adsorbent required for a given N2 adsorbate capacity (mol/g). The larger internal void fraction associated with increased porosity also increases the non-selective gas storage volume in the adsorbent bed and thereby decreases the separation capability, i.e reduces overall product recovery. Further, the crush strength of adsorbent particles is decreased with high porosity/low density adsorbent particles. This is a problem because adsorbent particles in the bottom of large commercial PSA beds must resist crushing under the weight of thousands of pounds of adsorbent contained in the adsorber vessel.
As a means of increasing zeolite content in zeolite adsorbents it is known to convert clay into zeolite via a process known as caustic digestion. It is also known that zeolite can be produced from preformed clay bodies, and that the shape of the preformed body can be retained.
Howell et al, in U.S. Pat. No. 3,119,660 disclosed a method of producing zeolite A, X and Y by forming kaolin clay into aggregates (also referred to as xe2x80x9cmassive bodiesxe2x80x9d) followed by calcination at 600-800xc2x0 C. and caustic digestion. They disclosed that this approach is particularly useful in aggregates having an increased clay content (in the range of 20 to 80%) because the greater the clay content, the more zeolite is formed cheaply.
Howell et al also disclosed that inclusion of a void forming, combustible diluent substance facilitates the clay to zeolite conversion, especially when the clay content is 50% or higher. This is because while clay is a non-porous material, zeolite is microporous, and therefore void space is needed for expansion with such clay to zeolite conversion.
The methodology of providing void space for volume expansion was further investigated by W. H. Flank et al (U.S. Pat. No. 4,818,508). They discovered that the rate of zeolite formation in massive bodies can be accelerated and the purity of zeolite enhanced by controlling the size of clay particles used to make the preformed bodies, together with addition of pore generating materials and inert binder.
Leavitt (U.S. Pat. No. 5,074,892) states that NaX adsorbent crystals may be treated with caustic to remove soluble, non-crystalline debris and enhance cation exposure.
S. M. Kuznicki et al disclosed a method to make X-type zeolite (U.S. Pat. No. 4,603,040) having a Si/A12 ratio of 2.0 (also referred to as xe2x80x9cmaximum aluminum Xxe2x80x9d). They extruded mixtures consisting of kaolin clay and 10 to 30% of a pore forming material into a preformed body. After calcining this material at 600xc2x0 C., the body was treated in an aqueous solution of NaOH and KOH. Typically such treatment converts meta kaolin into type A zeolite as well as a high purity maximum aluminum X zeolite (2.0) product. Unfortunately, in the example an aluminum zeolite X (2.0) could only be made by maintaining the treatment temperature at about 50xc2x0 C. for a period of 10 days.
Thus all the prior art related to caustically digested preformed X zeolite was directed to making a massive body of high zeolite content. Further, the materials formed were high density low porosity materials (as a result of high clay content and resultant low macropore volume), even with the use of organic burn-out.
The objective of this invention is to enhance the intrinsic mass transfer rate of PSA adsorbents while minimizing and/or eliminating the need for reduction in particle size. As a result, the materials of the invention can be used in PSA processes that have high adsorbent productivity and high process efficiency, short cycle times and low power consumption.
The invention preferably comprises an adsorbent material having an SCRR of greater than 1.2.
The invention preferably further includes a process for the separation of at least one first component from a gas mixture including said first component and a second less selectively adsorbable component using an adsorbent having an SCRR greater than 1.2.
The invention preferably includes a process of making an adsorbent comprising the following steps:
a) providing zeolite powder having a predetermined composition;
b) mixing said powder with a binder capable of being converted to zeolite via caustic digestion, wherein said binder is added in an amount less than 20% by weight, preferably xe2x89xa615%, more preferably xe2x89xa612% of the powder/binder mixture;
c) forming beads from said mixture;
d) calcining said beads;
e) caustically digesting said beads such that at least a portion of said binder is converted to zeolite;
f) recovering said adsorbent.
In further preferred embodiments, the process of making the adsorbent further comprises the steps of:
g) adding a combustible fiber or particulate material to the binder/zeolite mix prior to bead forming.